The invention relates to a steering booster configuration according to the preamble of claim 1.
EP 0 124 790 discloses a steering booster with electric drive which is installed coaxially in the steering shaft. Coupling-in of the servo power takes place with the aid of a servo motor which acts via a planetary gearing onto the steering shaft. The gearing and the motor are disposed coaxially with respect to the steering shaft, wherein the steering shaft is guided centrally through the configuration. Since motor and gearing are implemented as a separate assembled unit, the configuration becomes voluminous and comprises a large number of parts disadvantageously affecting economy and reliability. A particular disadvantage comprises that the sun gear of the planetary gearing is fastened on the rotor of the motor drive, wherein the rotor and the sun gear are disposed and rotatably supported coaxially with respect to the shaft axis. It is evident that in this implementation the sun gear always has a relatively large diameter, but can, at least, never have a smaller diameter than the steering shaft itself. Since the reduction ratio of a planetary gearing in the first step is determined by the ratio of the diameters of the sun gear and the planetary gears, it is readily evident that in this implementation large diameter ratios of the gears cannot be attained since in this structuring the sun gear itself always has a relatively large diameter. In the description of the above cited application a reduction ratio of 1:1 to 1:10 is specified. Greater reduction ratios would only be possible in the present embodiment if the overall gearing diameter were to be increased drastically, which would lead to unacceptable dimensions of the steering booster configuration.
Small gearing reduction ratios, in addition, have the disadvantage that the motor power must be correspondingly high and, furthermore, only slowly rotating motors can be employed which would demand corresponding expenditures. A further disadvantage herein is the less rapid response behavior which leads to reaction inertia in the system.
Due to the voluminous structuring less constructional space is available especially in the direction of the shaft axis, which primarily has a negative effect on the safety functions. In the event of an impact of the driver of the motor vehicle on the steering wheel, the energy must be absorbed as advantageously as is feasible in order for the motor vehicle driver not to be injured. This is attained in such a case thereby that the steering spindle is to yield by telescoping such that energy is absorbed in the course of telescoping. Thereon is based the requirement of making available sliding paths of greatest feasible length for the energy absorption device, which is not readily possible with servo drives known in prior art. The required large installation space in the axial direction thus comes at the expense of the desired energy absorption displacement path in the event of an impact.
A further disadvantage in known devices is that, due to the complicated structure, apart from mechanical play, they also have a certain reaction inertia as a result of moving large masses, which has a negative effect on the steering behavior and the economy. Complex [and demanding] structuring according to the known configurations, moreover, have the disadvantage of high energy consumption during operation. Apart from the requirements of economy for the provision of the requisite energy, additional expenditures are necessary in order to dissipate the corresponding heat losses.
It is the task of the present invention to eliminate the disadvantages of the above cited prior art. The task in particular comprises realizing an electric steering booster for a steering configuration which is constructed extremely compactly and which permits attaining rapid response behavior with good dynamics entailing responsive steering behavior for the driver, which operates at a high degree of efficiency and can be produced economically.
According to the invention the task is solved through the configuration according to the characterizing clause of claim 1. The dependent claims define further advantageous embodiments.
The task is solved according to the invention thereby that a disk-form electromotor is combined with a planetary gearing and both are installed in the shaft of a steering spindle, a steering shaft or a steering gearing pin, wherein the shaft in the region of installation of the drive unit is correspondingly interrupted along the steering shaft axis, or the steering shaft in this region encompasses an intermediate gear shaft which includes the sun gear of the planetary gearing. Since it is disposed in the steering shaft axis, the intermediate gear shaft can include a sun gear which is disposed in the steering shaft axis and rotates about the latter and allows very small diameters. The sun gear diameter in this design according to the invention is only limited by the required mechanical strengths. The sun gear diameter can in any case be only slightly greater, such as maximally the 1.25-fold of the steering shaft diameter. In order to attain especially high reduction ratios, the diameter can be of equal size or preferably smaller than that of the steering shaft itself. Very good ratios are attained if the sun gear diameter is in the range from 10 to 20 mm.
The configuration thus permits compact disk-form motor-gearing combinations which allow high reduction ratios. The disk-form motor-gearing configuration can herein preferably have a diameter of  less than 200 mm, preferably  less than 170 mm, with axial longitudinal extensions which are less than 90 mm, wherein reduction ratios from 1:10 to 1:40 are possible. The reduction ratios are preferably selected to be  greater than 1:20, wherein reduction ratios of  greater than 1:30 are even more favorable. The large reduction ratios lead to the fact that compact, economically operating, electronic disk armatures can be applied. Such disk armature motors comprise a disk-form rotor which comprises permanent magnets and is operated laterally to the disk face, preferably bilaterally, by electronically controlled stator windings. In order to be able to drive freely said intermediate shaft supported in the shaft axis, the intermediate shaft in the projecting region of the steering shaft is supported centrally rotatable and is driven by the rotor disk which comprises a hollow toothed gear and is coupled with a further gear rim of the intermediate shaft.
In order to be able to couple the motor power via the rotor onto the intermediate shaft, it is necessary to operate the rotor axis such that it wobbles slightly by a few millimeters, for example 3 mm, offset parallel with respect to the shaft axis during the rotation of the steering wheel. For this purpose the rotor is rotatably supported offset radially on the shaft axis, such that the hollow toothed gear of the rotor meshes only on one side with the gear rim of the intermediate gear. Since the stator winding is disposed stationarily in the housing configuration, the rotor in this configuration completes a movement wobbling in the radial direction by a few millimeters when the steering shaft is rotated and, driven simultaneously, rotates about the offset axis. This wobbling movement has the further advantage that the step-like movements, occurring in the case of disk armature motors which are commutated electronically, referred to as ripples, can be attenuated or compensated. Consequently, the power transmission in servo operation onto the steering wheel shaft is continuous and the undesirable ripple, or the rattle undesirable for reasons of safety, are absent. In order to keep further the overall diameter small, advantageously only two planetary gears meshing with the sun gear are employed. The planetary gears are rotatably supported in the driven-side projection of the steering shaft and comprise in the direction of their axis a further toothing with a diameter which, relative to the planetary gear toothing, is reduced. This further toothing meshes with a hollow gear with inner toothing, which is connected stationarily with the housing of the overall motor-gearing combination.
The entire configuration is operated upon actuation of the steering shaft via sensor elements, evaluation electronics and power electronics, such that, corresponding to the actuation of the steering shaft, a power coupling onto the steering shaft takes place and thus a smooth steering action is possible. The configuration provides a highly compact structuring which permits high efficiency and, moreover, can be well cooled with low expenditures. The required ergonomic steering behavior can be realized and, through the compact disk-form implementation, the necessary absorption elements can be optimally realized and installed for the event of a crash.